I played a 3v3 last night with mostly novice players. It was PTT vs. ZZT. One of my opponents got sight in the rear of our bases and started flooding the place with Roaches and Hydralisks via Nydus Worms.

We were able to fend off this attack with our various defensive units. Unfortunately, these had been up at the choke point and not really prepared to run back, which took some time so the Zerg were able do to some serious damage. What's more, the Zerg were able to retreat handily back to the Nydus Worm and live to harass again a minute later somewhere else.

What are some good ways to:

Detect an incoming Nydus attack quickly to react?

Destroy the incoming Nydus Worm before it completes building?

Take out the Nydus Worm after the units come out so they can't retreat?

Prevent the whole thing from happening in the first place?

In this case I don't think denying sight would be possible, considering they had a Terran player with no qualms about using Scanner Sweeps.

My initial thought for some defense was maybe getting some patrolling Marines or Reapers in the backs of the bases, just to start shooting at the thing when they saw it building. But then I wasn't sure how many it would take to soundly deny the building from completing.

Another thought I had was perhaps a Photon Cannon near the edge... would that be enough to take out the Nydus Worm before it completes (assuming we could place it well to see the best Nydus build locations)?

PS We did end up winning the game due to superior macro... and one of their players unfortunately disconnecting.

5 Answers
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Most of the advice to stop Nydus worms applies in general. First of all, you should be keeping an eye on your base no matter which race you are facing, and regardless of whether or not you have scouted a Nydus Network in the Zerg's base. All races have ways of getting units into the back of your base without going up the ramp. Medivacs, proxy pylons, overlords with ventral sacs, warp prisms, blink stalkers, colossi, reapers, etc, not to mention pretty much any air unit.

Denying vision to the zerg player by harassing overlords should be done in general, not just to stop worms.

To help detect the nydus worm (and all other threats mentioned in previous bullet), spread buildings out in your base, so you have vision everywhere. Terrans can spread supply depots, protoss have pylons, and zerg have creep tumours, or overlords to spread around.

Learn the sound a nydus worm makes when it completes. Sadly for us zerg players, the nydus worm announces itself to all players with a loud scream, similar to how everyone hears "nuke launch detected". On the plus side (for us), it only screams as it is finishing. This won't help you stop the worm, but it will alert you that you should be looking for the worm to make sure it's not in your base, or if it is, mobilize some troops to defend.

In team games, consider using the team colors option for the minimap, so your units are green and your opponents are red. This will make the nydus worm stand out in your minimap more than if you are orange and your opponent is yellow, or some other combo of similar colors.

Worms don't have a lot of HP, even when they complete. When they are building, a handful of units can kill it before it completes. I just did a quick test; 3 marines starting to shoot as soon as the worm reaches 50% done can kill it just after it finishes, enough time for only a few units to get out.

Remember that each worm costs the zerg player 100 minerals and 100 precious gas. Each time you kill one before it completes, the zerg is behind on resources. They can also only be building one at a time. (specifically, one per Nydus Network)

I will say that your base was harder to get into than your teammates' - you were quicker to bring even a couple of units to the Nydus as soon as you saw it.

Nydus worms, like most Zerg buildings, start with little-to-no health and gain it as they build (up to 200 HP), so an early response, even with just a few units, can make a significant difference. One of my later Nydus attempts into your base was destroyed right as it started in one hit from a Zealot, for example.

I wouldn't rely on Cannons because the Zerg player can almost always find a spot to build the Nydus worm that's out of range of the cannon, unless you spend so much on cannons that you're crippling your army.

What I learned from the campaign is that air units are very effective against Nydus Worms. Air units have the advantage of swiftly moving towards the Nydus Worm, without getting obstructed by the terrain.

So in the case of Terran, you'd use Banshees and keep them near hot spots where they are likely to pop up.

To prevent them from popping up, you first have to scout or scan their base to see if they actually have one. And if they do, spread out your air units as to reduce the traveling time towards the Nydus Worm. Hopefully, you will be able to destroy it before it releases (too many) units.

Also they have to be able to see the area to make it pop up, so scouting with a Raven to detect stealth or hidden units is advised. They can also use Overlords for this purpose and they are always a good target to take down on sight.

+1 for the Banshee idea. I did know they had the Nydus in their base, but I think preventing sight was pretty much impossible in this case due to Scanner Sweep so the only thing left is reactionary (or an all-out assault on their base Nydus).
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Mag RoaderSep 9 '10 at 14:53

It was pretty early in the match for anybody to have a significant air force, for what it's worth.
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lilserfSep 9 '10 at 14:55

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@lilserf, you only need one or two to effectively kill it. Once you have the initial group, you'll only get increasingly stronger against them. Plus Banshees are also useful against Roaches, Zealots and Marauders (all can't shoot in the air)
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Ivo FlipseSep 9 '10 at 15:00

Sure... I just think it's probably more efficient to respond quickly with cheaper units than to tech all the way up to Banshees.
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lilserfSep 9 '10 at 15:05

When I play Zerg I love to abuse the Nydus worm. The two things that screw me up are:

Denying vision, usually through killing my Overlords. Obviously it's impossible to completely deny vision to the enemy (especially if their allies have Observers or comm sweeps), but often I will loose Overlords to people who were smart with turrets or air units. A few of these, or even a marine or two in back of the base, will go a long way towards discouraging invasions.

Destroying the worm, usually as it's spawning. The Nydus attack is all about sneaking in. If the enemy spots it early, then it's toast. It's vulnerable while building, and shortly thereafter since units can only come out of it one at a time. I have seen garrisoned units make short work of worms, as well as Protoss units warped-in ASAP, and even workers attacking the worm.

A good Zerg Nydus player will distract you with an attack or raid, and then find a place to put the worm where you won't notice it and have trouble getting to it quickly. But by exercising some of the same tricks you use to discourage drops, reaper raids, air harassment, and so on, you can be more prepared.

As was mentioned in the question, if the zerg player has an ally with scanner sweeps, you can't do anything about the vision
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BlueRaja - Danny PflughoeftSep 9 '10 at 20:37

Even with sweeps, I think you can deny vision. With just sweeps, the Zerg only has vision for a moment, when the Terran doesn't need a MULE or sweep elsewhere. So I'd still say try to deny vision... if you ignore it, the Zerg can park Overlords near you, and pick the perfect location and timing for the Nydus attack.
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WikwocketSep 9 '10 at 21:19

Not to mention that a scanner sweep is very visible if you're paying attention and only lasts for a while. If they're using that to put the nydus worm in, you know exactly where it's going to go.
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wdsSep 10 '10 at 11:14

Of course we would place the scanner sweep at a ridiculous edge of the map, so only a corner of it would reveal a tiny part of the base. So far that hardly every gets noticed and we can baneling bust a main 80% of the time without any resistance. ^^ Quite potent.

Air may seem the best counter, but since we also just build anti-air by default ('cause we see people switch air when applying this tactic), we're just happily ready with our anti-air force ^^